Video Credit: Courtesy Capt. Billy Deans
***NOTE:*** This video was recorded over 20 years ago.

The Witches Brew – Trimix and Nitrox Gas Mixtures

So you’re planning on going diving this weekend and need to get your cylinders filled with nitrox. Imagine that you have a choice of getting that mix from one of only six dive centers in the United States. You can go to Long Island, New York; Gloucester City, New Jersey; Key Largo or Key West, Florida; Santa Cruz, California; or Bellingham, Washington to get those tanks filled. That’s right, back in the early days of technical diving we had a choice of six dive centers in the US that would fill our tanks with oxygen enriched air. If you were an active nitrox diver, the ability to obtain breathing gas was very limited. Getting gas required logistics far beyond that of driving to the local dive shop. Yet, that did not stop people from getting the coveted breathing gas. If you were near these dive centers, it was easy. If not, creativity crept in.

Tell any diver they cannot have something and soon they will make it themselves. It did not take long for divers to start mixing gas in their garages. Dive shop owners were strongly against this practice, and the industry frowned upon it just as loudly. Yet divers were mixing gas at home more than most could have imagined. Some divers were very successful at home brewing, but others were sloppy and dangerous. Most anyone with simple math skills could calculate proper proportions of oxygen and air to make a nitrox mixture. However, mixing nitrox gas was more complex than transfilling some oxygen and topping it up with air. It required oxygen, hoses, fittings, gauges, flow meters, an oxygen analyzer, and an air compressor. In addition, it took an understanding of cleaning for oxygen-service, gas system design, heat exchange, and the dangers of adiabatic compression. Most people could put the first parts together, but it was the oxygen compatible air and oxygen cleanliness that were the missing links for many. Some divers would fill their scuba cylinders at home with a specific amount of oxygen and then have them topped off with air at the local dive shop. On the surface, this seemed a logical solution, but it violated every rule of gas mixing, and it put many people at undue risk. In some situations, it resulted in serious accidents. If they were cutting corners in one part of this process, what else was not as good as required?

For divers who wanted to do it right, they sought out training from experts like former NOAA Diving Officer, Dick Rutowski in Key Largo, Captain Billy Deans in Key West, or ANDI co-founder, Ed Betts on Long Island.. The courses offered were comprehensive and usually took two to four days to complete. For those who could not get to these rare training events, the early CompuServe internet forums were a major source for gas mixing discussions. In addition, divers would share information they garnered from the Navy and NOAA diving manuals, as well as from the commercial diving industry. Along with all the seriousness and challenges that were facing technical diving in the early days, deep diving pioneer, Capt. Billy Deans created a video that parodied decompression table creation, breathing gas analysis and gas mixing. It gives today’s divers and gas mixers an idea of what we were up against back in the early days of recreational mixed gas diving.

It was not until Technical Diving International introduced the first TDI Gas Blender Course and textbook written by Jan Neal, that gas-mixing methodology became readily available to divers worldwide. Neal’s materials legitimatized recreational gas mixing and formed the basis for what we do today in teaching divers about mixing gases for diving. The dive industry as a whole has benefited from the contributions of divers over the years to gas mixing.

Learning about mixing gas is much easier today than it was twenty years ago. A whole industry has emerged that has made the tools of the trade much easier to get; between training, gas analyzers, gas boosters, compact compressors, filtration systems and a host of other items. While divers are able to get gas mixes from most any dive center around the world, there are those who still like to mix on their own and the right stuff is easy to get. Like in most every industry, great ideas and techniques are the product of a few brave individuals who sought out knowledge and adventure, many times from the workbench in their garage.

Joel Silverstein is the author of three books on nitrox, a major contributor to the NOAA Diving Manual, and TDI Instructor Trainer # 125. Since 1989 he has mixed more than two million cubic feet of breathing gas. Contact: joel@techdivinglimited.com

Labeling and Identifying Nitrox Cylinders

Any time you fill a tank with nitrox, it must be identified as such. This will help to prevent accidents in the event that someone uses a tank filled with nitrox without taking the proper precautions. The industry standard for tanks filled with nitrox is to mark the tank with a tank wrap, as well as identify the mixture with a label or tag.

A nitrox tank wrap is an adhesive decal, generally 10-13 cm. (4-5 in.) in width, that is designed to completely encircle the diameter of the tank. The decal is usually printed in yellow and green with the word Enriched Air and/or Nitrox printed continuously on it in bold green or yellow letters. This tape makes it quick and easy to spot a nitrox bottle in a group of tanks.

It is standard practice to identify the actual nitrox mix currently contained within the cylinder and note it on a Contents label. This information can be found by analyzing, or watching someone analyze, the cylinder for oxygen content using a properly calibrated and functioning analyzer. At a minimum the information recorded on the label should include; oxygen content, maximum operating depth (MOD) of the gas you will be breathing, the name / initials of the person who analyzed the mixture, and the date it was analyzed.

After you analyze the cylinder and find the oxygen content, the rest comes pretty easy. The only additional “how to” necessary is calculating the Maximum Operating Depth (MOD).

To calculate the MOD for a specific ppO2 and percentage of oxygen (FO2) the following formula is used:

The additional information on the Contents label concerning nitrogen and helium is primarily intended for use by certified TDI Technical Divers trained to dive with helium in their breathing gas mixtures.

Finally, it is important to note that although the industry standard is described and pictured here, some countries require different looking labels by law. A proper nitrox diver course will teach you these procedures and allow you to practice until you are comfortable analyzing and properly labeling a nitrox cylinder.

People make mistakes, it’s human nature. I make them all the time. I’m sure that even after this article has been edited several times and published someone out there will find at least a couple of typos and call us out on it. A typo is one thing. However, a simple mistake in the blending process can result in a diver breathing a mix with significantly more or less oxygen than they had expected, causing serious injury or death. If we KNOW that people make simple mistakes so often, then why do so many nitrox divers today NOT analyze their gas before diving? There are two primary reasons: either they don’t understand why it’s so important (a topic that is covered in every nitrox course), or they have just become complacent. This article will discuss both scenarios and how to avoid them.

Why is it so important to analyze your breathing gas? Simply, it can kill you if it’s wrong. If the oxygen content is less than the diver had expected, they can end up with unexpected and unknown decompression obligations.

Example – You make a dive to 30 metres/100 feet assuming you’re breathing 32% nitrox. You spend 39 minutes on the bottom and surface with no decompression obligation. Unfortunately, the nitrox tank you were diving was accidentally filled with air (21% oxygen), and in reality you just blew off 26 minutes of decompression. A significant error that is almost sure to result in Decompression Sickness. This situation can be made significantly worse by conducting repeated dives.

What if the oxygen content is HIGHER than you expected? Should be better off then, right? As far as decompression obligations are concerned, yes. However, a far greater risk in diving nitrox is Oxygen Toxicity and can cause severe convulsions (not a good situation underwater).

Example – Using the same dive as above, assuming you were on 32% nitrox at 30 metres/100 feet, your partial pressure of oxygen (PO2) would be close to its upper limit at about 1.3 ata. If that nitrox mix was in fact a 50% nitrox mix, your PO2 would now be over 2.0 ata and would be considered extremely dangerous.

The examples above are not the only concerns of breathing the wrong gas at the wrong depth; a thorough nitrox course will cover the others, as well as how to avoid them. So if you have to be Nitrox certified to dive nitrox, and the risks and proper procedures for avoiding those risks are covered in the course, why do people still end up breathing the wrong gas? The simple answer is: complacency. Over time, divers become complacent with their gas analysis procedures and start to skip it altogether, which means they end up in the water with absolutely no idea what they are breathing. Pretty scary.

Normalization of deviance is a term used by astronaut Mike Mullane (*Mullane 2014) to describe the process of complacency in safety procedures. In brief, it explains how humans have the tendency to take shortcuts due to different factors including time, peer pressure, etc. Once this shortcut is taken and nothing bad happens, the brain will incorrectly assume that the shortcut is “safe”. This shortcut now becomes the norm, and we have completely eliminated a critical step in a procedure. This applies to diving at every level. How many times have you seen divers jump in the water without doing a proper predive check? It is taught and its importance stressed in every open water course, yet it gets skipped every day because so many divers have “gotten away with it” they assume it’s safe to dive without making predive checks and then eliminate it from their procedure. Unfortunately, it also results in emergencies from divers forgetting to turn on their air and inflate their BCDs.

The same happens to nitrox divers. Maybe one day they are in a rush and forget to analyze their gas at the fill station. They get to the dive site and realize that they forgot to analyze but now do not have access to an analyzer. They are left with two choices, either not dive today or dive without analyzing their gas. The diver has been getting fills from that fill station for years and has never gotten the wrong mix, so they decide to dive anyway and assume the fill is correct. Nothing bad happens, so they now believe this shortcut is safe. “If I get my fills from XZY Dive Center, I know that it will be correct and I do not need to analyze my gas”. They have eliminated the most critical step in diving nitrox, and this is now the norm.

We know people make mistakes, and that’s why we have safety procedures in diving. These procedures help us catch the little mistakes before they create catastrophic emergencies. When diving nitrox, analyze every tank before every dive without exception. It could save your life.